This application relates to a system and method for determining the thickness of a brake lining based on cooling wave propagation time.
Most motor vehicles include a brake system having a set of brake shoe assemblies for retarding the rotation of the wheels of the vehicle when the brakes are applied. Typically, each brake shoe assembly includes a brake lining made of a friction material which gradually wears away during brake applications. After numerous brake applications, the brake lining wears below a critical material thickness and, therefore, should be replaced. As a result, the brake lining must be periodically inspected for excessive wear. Thus, it is desirable to provide an indication when the brake lining thickness is worn such that the brake shoes should be changed.
Several problems arise when trying to determine whether the brake linings have sufficiently worn such that they need to be changed. Often, the wheel must be removed from the vehicle so that the brake lining thickness can be measured. This is cumbersome and time consuming. Visual brake lining wear indicators, such as notches in the lining or color coded layers in the lining, have been used to determine when the linings should be changed. When a visual wear indicator is used, an inspector can visually examine each brake lining to determine whether the brake lining requires replacement without physically measuring the thickness of the brake lining material, However, the use of visual wear indicators can also be cumbersome and time consuming because visual wear indicators require the inspector to visually check each lining while the vehicle is stationary. Thus, it is desirable to have an efficient way to monitor the brake lining thickness during the operation of the vehicle to indicate whether the brake linings require replacement without having to visually inspect each brake lining.
The assignee of this application has recently developed systems that monitor brake lining thickness during the operation of the vehicle by using a brake lining including a single temperature sensing device. Typically, the electrical resistance of the temperature sensing device changes with the temperature of the brake lining. In these systems, the thickness of the brake lining is determined by monitoring the increase or rise in temperature of the brake lining during a brake application. During each brake application, a "heat wave" is produced on the surface of the brake lining. The amount of time it takes for the "heat wave" to travel from the surface of the brake lining to the temperature sensing device embedded in the brake lining is measured and recorded. This time period is commonly referred to as the heat wave propagation time. The thickness of the brake lining is a function of the heat wave propagation time and, therefore, the brake lining thickness may be calculated using the measured heat wave propagation time. In general, a long heat wave propagation time indicates a thick brake lining and a short heat wave propagation time indicates a thin brake lining. However, during certain braking conditions, such as light braking applications, both the braking duty cycle and the amount of brake pressure applied during the brake application affects the heat wave propagation. That is, the force applied on the brake shoe and duty cycle of the application may affect the temperature rise and, therefore, the time it takes to reach a threshold temperature of the sensing device. Thus, it would be possible to improve upon this type of system, independent of these input variables.